Prototyping is a vital step in product development. Numerous technologies have been discovered to churn out prototypes effortlessly, economically, more importantly, rapidly. In recent times, a technology known as rapid prototyping has come under the spotlight.

Article Description

The term rapid prototyping (RP) refers to a class of technologies that can automatically construct physical models from Computer-Aided Design (CAD) data. These "three dimensional printers" allow designers to quickly create tangible prototypes of their designs, rather than just two-dimensional pictures. Such models have numerous uses. They make excellent visual aids for communicating ideas with co-workers or customers. In addition, prototypes can be used for design testing. For example, an aerospace engineer might mount a model airfoil in a wind tunnel to measure lift and drag forces. Designers have always utilized prototypes; RP allows them to be made faster and less expensively. Rapid prototyping produces a physical three-dimensional plastic model, using CAD file data through special sintering, layering, or deposition techniques. This offers tremendous advantages in terms of lower cost of prototyping, shorter lead times, and improved total product quality.

Prototypes play an important role in the process of product development. Traditionally metals were either machined or cast into the desired shapes. Both the processes, however, require skilled manpower and are costly, time-consuming operations. Today, plastics prototypes are gaining popularity. Designing and making the prototypes of plastic products, is one of the most challenging tasks faced in many industries. The plastic prototype becomes very important for ensuring one go product as the production moulds for plastics are very expensive and require considerable fabrication time.

Rapid prototyping
During last decade, many new technologies were developed for rapid prototyping. A physical three-dimensional plastic model is created, using CAD file data through special sintering, layering, or deposition techniques. This offers tremendous advantages in terms of lower cost of prototyping, shorter lead times, and improved total product quality. In recent years, many reputed magazines like Mechanical Engineering, Journal of Engineering Manufacture, Modern Plastic International, etc have started publishing success stories of rapid prototyping (RP) technologies. One such by Norbert Mucheyer, president of German prototyping service, Mucheyer Service, explains, "Typical mould for a part like telephone housing costs about $ 60,000 (Rs 21,00,000) and reworking on mould design mistakes often add 20 - 30 per cent to the cost of machining. Generating RP parts by stereo-lithography or SLA from simple drawing typically cost about $ 1500 (Rs 55,000), but eliminates extra cost of reworking on the moulds. This does not include benefits derived from advanced inputs to marketing and production teams, which results in drastic reduction in product development time, which are substantial." (Source: Modern Plastic International, Sept 1993)

All RP technologies depend on fully defined geometry of complex parts in computer-aided design. This three dimensional CAD model is split into a series of two-dimensional slices. Rapid prototyping system using this data, then builds three-dimensional (3D) part in layer after layer. Today, there are about ten companies offering RP systems, and these can be classified in two groups -
o Laser or light processing of material
o Material deposition techniques
In the first approach:
o UV laser is used for cuing photosensitive resin in stereo-lithography technique (SLA) developed by 3D Systems Inc
o UV laser is used to sinter thermoplastic powders or polymer coated metallic powders selectively, in selective laser sintering (SLS) process commercialised by DTM corporation
o Tin layers of sheets are laser cut in laminated object manufacturing process (LOM) developed by Helisys Inc
o UV light is used to cure acrylate polymer through a photo mask to solidify entire layer in solid ground curing (SGC) marketed by Cubital Ltd
o Light sculpting (LS) is very similar to photo masking developed by light sculpting Inc

In the second approach, resistance heating is used to melt nylon, ABS plastics or investment casting wax and a miniature pump extrudes liquid material through a nozzle and one layer is drawn - up at a time like in XY plotter; in fused deposition modelling (FDM) marketed by Stratasys. Ballastics Particle Manufacturing (BPM) uses ink-jet nozzle principle to deposit droplets of molten material, marketed by BPM Technology Inc.

Recent development in RP is in 3D printing where electrostatic ink-jet of binders is sprayed onto layers of silica or alumina. These layers are bound together to form the 3D shape, which is further fired for fussing and strengthening the part.

Laminated object manufacturing (LOM)
LOM was developed and commercialised by Helisys Corp, USA. LOM builds shapes with layers of paper or plastic. The sheet is available in the form of a roll with a thermally activated adhesive on one side. The paper roll indexes by a constant distance over a table every time, where the part is being built. Using a heated roller, this new layer, or laminates is glued to the previous layer. A laser cuts the outline of the part cross-section for each layer. The strength and focus of the laser is such that the depth of cut is equal to the laminate thickness. The schematic diagram of the process is shown in Figure1. The laser then scribes the remaining material in each layer into a crosshatch pattern of small squares. As the process repeats, the crosshatches build up into tiles of the support structure. The crosshatching facilitates removal of this tiled structure when the part is completed. The laser spends hardly five per cent of the time in cutting the contours of the part, while the remaining 95 per cent of the time is wasted in cutting the stock. LOM builds large parts relatively faster because only contours are scanned. Internal cavities are hard to form with LOM, since it is difficult to remove the sacrificial material from the internal regions.

Advantages: The advantages of this technology can be listed as follows-
o Only the outline is cut and no time is spent in building the interior of the layer. Therefore, this process is fairly fast
o The materials used for building the parts (wood, paper, etc) are the least expensive among all RP processes
o Cost of the machine is one of the lowest
o No external support structures or post-curing is required
o It is suitable and economical for making large parts to be used as patterns for sand castings
o The process can be carried out unattended
o LOM is also a direct rapid tooling process. It has been successfully used in making metallic laminated tools for sheet metal forming operations.

Limitations: The disadvantages of this technology can be listed as follows-
o Parts are weak along the z-direction
o Paper parts have poor surface finish and absorb moisture
o The process is not suitable for making small intricate parts. As the stock needs to be chipped out during de-cubing, a fair amount of skill is required
o There is a lot of material wastage.

Stereo-lithography (SLA)
Stereo-lithography systems build shapes using light to selectively solidify photo-curable resins called photo-polymers. It is currently the most widely used RP technology and was first commercialised by 3D Systems, USA. Stereo-lithography creates acrylic or epoxy parts directly from a vat of liquid photo-curable polymer by selectively solidifying the polymer with a scanning laser beam. Parts are built up on an elevator platform that incrementally lowers the part into the vat by the distance of the layer thickness. To build each layer, a laser beam is guided across the surface, by servo-controlled galvanometer mirrors, drawing a cross-sectional pattern in the XY plane to form a slice. The platform is then lowered into the vat and the next layer is drawn, which adheres to the previous layer. These steps are repeated, layer-by-layer, until the complete part is built up.

In the SLA machine, complete polymerisation does not take place. Therefore, after all layers are built, the excess liquid polymer is drained and the prototype is put in a separate chamber with a flood of light to complete the polymerisation. Features with gradually changing overhangs can be built without support structures. The buoyancy of the viscous raw material supports the layer to some extent. However, large overhanging features require supports, which are typically thin wall sections or bristles-like structures that can easily be broken or cut from the part upon completion.

QuickCast is a SLA process from 3D Systems where the part is made hollow with an interior honeycomb structure. This is used as a consumable pattern in shell casting.
Advantages: The advantages of using this technology are:
o Accuracy of ± 0.